Technical Field
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device.
Background Art
Wide-bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), and diamond (C) are expected to have numerous applications, particularly in power devices, due to having excellent performance characteristics such as high dielectric breakdown field strength and high thermal conductivity. Among these, SiC in particular has attracted attention due to allowing oxide films (SiO2) to be formed using thermal oxidation processes, similar to when working with pure silicon (Si).
Semiconductor devices that use wide-bandgap semiconductors exhibit higher dielectric breakdown field strength than those that use Si. For example, 4H—SiC, GaN, and diamond respectively make it possible to achieve dielectric breakdown field strengths of approximately 10, 11, and 19 times greater than with Si. For a device of a given breakdown voltage, this makes it possible to increase the impurity concentration and decrease the thickness of a low concentration n-type (n−) drift layer, thereby making it possible to achieve a high breakdown voltage and a low on-resistance.
If the SiC body diode is used as the path for this reverse current, the on-resistance increases (this is a well-known phenomenon).
This increase in on-resistance is thought to be due to an increase in the portion of the current path through which it is difficult for current to flow that occurs when a forward current flows across the body diode after the conductivity is modulated (see Non-Patent Document 1, for example). The specific reason behind this increase in the portion through which it is difficult for current to flow is thought to be the formation of stacking faults in the crystal structure of the SiC due to the recombination energy of the majority carriers and the minority carriers.
One method of preventing current from flowing through the SiC body diode is to allow current to flow through the channel of the MOSFET, for example. However, switching ON the switching elements in both the upper and lower arms at the same time can cause a short-circuit in the power supply. Moreover, switching OFF some of the switching elements in order to prevent multiple switching elements from being ON at the same time results in an increase in OFF time (or so-called dead time). Furthermore, a forward current will still flow through the SiC body diode during this dead time.
Another method of preventing current from flowing through the SiC body diode is to connect diodes (Schottky diodes) in parallel with each switching element. However, if the forward voltage across these diodes becomes greater than or equal to the built-in voltage of the body diode of the switching element (which is approximately 2.3V for SiC), current begins to flow through the SiC body diode. This creates a need to reduce the forward voltage Vf of the diodes, which typically makes it necessary to prepare larger-area diodes and results in an overall increase in cost.